This invention relates to a movable frequency-domain induction apparatus and method for use in locating subsurface objects.
It is important during excavation to have a precise map of the subsurface so as to avoid damaging existing utilities such as water, gas, and electric lines. For example, when new pipes are to be installed in a certain area, the location of any existing pipes in that area must be known to avoid damage to them when the trenches for the new pipes are being excavated. The lack of accurate subsurface maps for construction sites results in thousands of broken pipes and billions of dollars in repair costs each year.
Most currently used pipe location equipment requires an operator to connect a transmitter directly to the pipe. With this equipment, the attached transmitter injects a single-frequency current into the pipe at a location where the pipe is exposed. For example, with a water pipe the current may be injected at a fire hydrant. The resulting magnetic field on the surface is then measured with a single hand-held receiver that records one or several of the components of the magnetic field. If only a single straight pipe is present, the hand-held receiver can accurately determine the direction and depth of that pipe. However, the pipe must be exposed so that the transmitter can be connected. Unexposed pipes cannot be located using this equipment.
There are numerous obvious advantages to pipe location equipment that does not require an operator to directly connect a transmitter to the pipe. For example, such equipment would permit one to locate unexposed pipes and would not require any direct physical connection. Such pipe locating equipment generally relies on either induction or wave propagation. Wave propagation is used in ground penetrating radar. However, such wave propagation methods require the soil conductivity to be relatively low. When the soil conductivity is high, the radar waves attenuate rapidly and may fail to reach the subsurface regions that contain the pipes. By contrast, induction methods work well when the soil conductivity is high.
To understand why induction methods work well in situations where radar methods may fail, consider a plane wave that propagates in a homogeneous soil with permeability xcexc0=1.256xc3x9710xe2x88x926 H/m, relative permittivity xcex5r, and conductivity "sgr". Assume that the plane wave is time harmonic with angular frequency xcfx89=2xcfx80f. If the direction of propagation is the x axis, then the plane wave behaves as exp(ikx) where the propagation constant k is given by (xcfx892xcexc0xcex50xcex5r+i xcfx89xcexc0"sgr")xc2xd with xcex50=8.85xc3x9710xe2x88x9212 F/m. The decay of the plane wave is then given by exp(xe2x88x92Im(k)x), where Im(k) denotes the imaginary part of k. For typical soils, xcex5r ranges from 2 to 30 and "sgr" ranges from 10xe2x88x926 S/m to 1 S/m. To illustrate the difference between radar and induction methods, assume that the soil is wet clay for which xcex5r=10 and "sgr"=0.2 S/m. Typical center frequencies for radar systems and induction systems are 400 MHz and 20 kHz, respectively. Using these values, it follows that the decay of the plane wave is exp(xe2x88x92(11.0/m)x) for the radar signal and exp(xe2x88x92(0.13/m)x) for the induction signal. Thus, for every meter of propagation the radar signal decays 95 dB whereas the induction signal decays 17 dB, and hence only the induction method could be used in such soil to locate pipes buried more than a few centimeters under the surface.
A wide variety of induction systems are commercially available. Each employs loop antennas, and they range in size from hand-held devices (such a simple metal detectors) to large systems, such as those used for mining applications, that have loop sizes on the order of hundreds of meters. These induction systems are designed primarily for the detection of conductive ores, aggregates, aquifers, bedrock, and buried waste.
One such commercially available system is the EM31 that is sold by Geonics Limited, which provides numerous induction tools for geophysical exploration. (The Geonics products are described in U.S. Pat. Nos. 4,070,612 and 4,199,720.) The EM31 is a handheld device that consists of a transmitter loop and a receiver loop separated a fixed distance from each other. The loops are small enough to be approximated by magnetic dipoles, and the tool can be operated in both vertical and horizontal dipole modes. However, since the EM31 has only a single receiver, it is cumbersome to use it to collect densely-sampled data over a large survey area. Further, only a single transmitter-receiver spacing (3.7 meters) is achieved with this tool.
Geonics also produces the EM34-4 system, which consists of transmitter and receiver coils and which can be operated at three operator-selected spacings. This variable spacing permits the device to be used at different exploration depths. However, to create a data set with more than one transmitter-receiver spacing with this device, one would have to repeat the survey with different coil separations. Further, with each pass, data is collected along only one line, which means that to cover a large area one must bring the device back and forth many times. Thus, the EM34-4 is not an efficient tool for collecting densely-sampled data for large survey areas.
The present invention provides a frequency-domain induction system that includes a movable array containing two or more spatially separated receiver array elements and one or more spatially separated transmitter array elements, transmission circuitry for transmitting time-harmonic electromagnetic fields over the transmitter array elements, and reception circuitry for detecting over the receiver array elements magnetic fields induced by the time-harmonic electromagnetic fields.
In one embodiment of the present invention, the movable array is attached to a movable cart. In another embodiment, the movable array contains an equal number of receiver array elements and transmitter array elements. In yet another embodiment, each of the receiver array elements is integrated with one of the transmitter array elements. In a further embodiment, each of the receiver array elements is spatially separated from each of the transmitter array elements.
In one embodiment of the present invention, the receiver array elements are collinear, and, in another embodiment, the transmitter array elements are collinear. In a further embodiment, the transmitter array elements and the receiver array elements are collinear.
In one embodiment of the present invention, the transmission circuitry simultaneously transmits time-harmonic electromagnetic fields having different frequencies. In another embodiment, the transmission circuitry sequentially transmits time-harmonic electromagnetic fields having the same frequency. In a further embodiment, the transmission circuitry transmits time-harmonic electromagnetic fields of at least two different frequencies and the reception circuitry detects each frequency over each receiver array element.
In one embodiment of the present invention, the spatially separated receiver array elements are uniformly spaced. In another embodiment, the spatially separated transmitter array elements are uniformly spaced.
The present invention also provides a method for detecting subsurface objects, comprising the step of providing a movable array containing two or more spatially separated receiver array elements and one or more spatially separated transmitter array elements, the step of transmitting time-harmonic electromagnetic fields over the transmitter array elements, the step of detecting over the receiver array elements magnetic fields induced by the time-harmonic electromagnetic fields, and the step of analyzing the magnetic fields to detect the presence of a subsurface object. One embodiment of the present invention includes the additional step of moving the movable array along a path above subsurface objects. In yet another embodiment, that path is a straight path.
In one embodiment of the present invention, the transmitting step further comprises simultaneously transmitting time-harmonic electromagnetic fields of different frequencies. In another embodiment, the transmitting step further comprises sequentially transmitting time-harmonic electromagnetic fields of the same frequency. In a further embodiment, the analyzing step detects the presence of a subsurface conducting object, and, in an additional embodiment, the analyzing step detects the presence of a subsurface non-conducting object.
The present invention further provides a frequency domain induction system for locating subsurface objects comprising a movable cart, an array attached to the movable cart and including two or more spatially separated receiver/transmitter pairs, transmission circuitry adapted to transmit time-harmonic electromagnetic fields through the transmitters, reception circuitry adapted to receive magnetic fields induced by the time-harmonic electromagnetic fields, and analysis software capable of analyzing the magnetic fields to detect the presence of subsurface objects.